Hydraulic pump or motor



April 7, 1964 R. w. BRUNDAGE 3,127,843

HYDRAULIC PUMP OR MOTOR Filed March 22, 1960 ATTORNEY rates This invention pertains to the art of hydraulic pumps or motors and more particularly to a hydraulic pump or motor of the positive displacement type.

This application is in many respects a continuationin-part of my co-pending patent application Serial No. 613,235 filed October 1, 1956 now Patent No. 3,011,447, issued December 5, 1961 and describes and claims subject matter non-elected in such application as a result of requirement of election by the Examiner. This application is also a continuation-impart of apparatus described in my co-pending application Serial Nos. 656,- 657 filed May 2, 1957 now Patent No. 2,956,512 issued October 18, 1960, 656,117 filed April 30, 1957 now Patent No. 3,007,4l8, issued November 7, 1961 and 814,- 320 tiled May 19, 1959 now Patent No.3,034,448, issued May 15, 1962.

The invention is particularly applicable to what is generally known as internal gear type pumps and will be described with particular reference thereto, although it will be appreciated that the invention in some of its aspects has broader applications, and in many instances may be applied to internal gear type motors or to vane or rotating cylinder type hydraulic pumps or motor.

Furthermore, the present invention is particularly applicable to hydraulic pumps or motors operable at what may be termed very high hydraulic pressures; that is to say, above 1,000 pounds per square inch and often times approaching or exceeding 4,000 pounds per square inch. At such pressures, constructions and expedients usable at the lower pressures are often unsatisfactory and inapplicable to the problems where the higher pressures are encountered.

Internal gear-type hydraulic pumps are normally comprised of an internally-toothed and an externally-toothed gear member rotatable on spaced axes in a housing with the teeth of the gears in sliding, sealing engagement. The externally-toothed gear is supported on a shaft rotatably mounted in the housing. The internally-toothed gear in turn is mounted for rotation on an axis spaced from that of the shaft by means of an eccentric bearing. Sealing members engage the axial faces of the gear members so that when the gear members rotate, they will define a plurality of closed chambers which revolve about the axis and progressively increase in volume to a point of maximum volume which corresponds to the point of open mesh of the gears and then decrease to a point of minimum volume which corresponds to the point of closed mesh of the gears. Normally, in a pump, the chambers which are decreasing in volume communicate with a discharge port and are at high hydraulic pressures while the chambers which are increasing in volume com municate with an inlet port and are at relatively low pressures. These high hydraulic pressures are unsymmetrical in the pump and exert large unsymmetrical radial and axial forces on the gears, sealing members and bearings which create problems with which the present invention primarily deals.

One of such problems has been to support the shaft for rotation in the housing under the very high radial forces on the inner gear member due to the hydraulic pressures in the decreasing volume chambers. Heretofore a pair of short lived needle bearings, one at each axial end of the gears, have been employed. By the present invention, one large roller bearing is employed and the other hearing may be a conventional inexpensive sleevetype bearing.

This same radial force is transmitted to the housing at points axially spaced from the centerline of the force and heretofore such force has been of sufficient magnitude to exert a bending moment on the drive shaft which has adversely affected the alignment of the members in sealing engagement with the axial ends of the gears. The present invention also deals directly with this problem.

Thus, in accordance with the invention, one of the sealing members is formed so as to in effect he an integral part of the drive shaft. The shaft. and sealing member are arranged to have limited axial movement relative to the gear members and the sealing member is biased against the axial face of the gear members by an axial force greater than the radially offset axial force thereon. The result is tht the sealing member contributes substantially to the bending strength of the drive shaft and also the radially offset axial forces create a turning moment which is transmitted to the shaft not only to oppose the bending moment of the radial forces on the externally-toothed gear but to reduce the radial loading on one of the hearings to the point that a sleeve type bearing may be employed at Very high hydraulic pressures.

Furthermore, the sealing member is mounted for rotation in the housing in a bearing member having a high radial load carrying characteristic and the centerline of this hearing is located sufliciently close to the axial centerline of the gear members that the bearing member will accept and carry a major portion of the radial load on the shaft whereby a bearing having a limited radial load carrying capacity may be employed on the side of the gears remote from the sealing member to carry the resilient load not transmitted to the housing through the high capacity bearing.

In order to hold the sealing member in engagement with the axial end of the gears, the axial end of the sealing member remote from the gears forms a pressure surface exposed to the high hydraulic pressures. However, it is necessary to limit the area of this pressure surface, otherwise excessive sealing forces will result.

In accordance with the present invention, a sealing ring is provided which is axially slidable in and in sealed relationship with the housing. This ring has an axially facing sealing surface in sealing engagement with the pressure surface of the sealing member and an opposite facing pressure surface of a limited area exposed to the high hydraulic pressures. The axial hydraulic force thus created provides a pressure between the sealing member pressure surface and the ring sealing surface sufficient to prevent the leakage of high pressure hydraulic fluids therebetween.

The principal object of the present invention is the provision of a new and improved hydraulic pump or motor of the general type described which is capable of having high hydraulic and volumetric efiiciencies and. is relatively simple and economic to build.

Another object of the invention is the provision of a 'new and improved arrangement for counteracting the effects of the high hydraulic forces on the various members of the pump or motor.

Another object of the invention is the pro-vision of a new and improved device of the class described wherein the parts are so arranged that only one high radial load carrying capacity roller bearing is required and the other hearing may be of a simple low radial load carrying characteristic.

Another object of the invention is the provision of a new and improved arrangement for resisting the bendu! ing effects on the drive shaft of the radial hydraulic forces on the externally-toothed gear.

Still another object of the invention is the new and improved arrangement for limiting or reversing the loading on one of the bearing members of the device.

Still another object of the invention is the provision of a new and improved arrangement for limiting the area of the sealing member exposed to the high hydraulic pressures within the housing.

Another object of the invention is the provision of a new and improved shaft sealing arrangement for rotating members which is simple in construction, has a minimum of friction and is highly effective.

The invention may take physical form in certain parts and arrangements of parts, the preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which are a part hereof and wherein;

FIGURE 1 is a side cross sectional view of a hydraulic pump illustrating a preferred embodiment of the invention, the section being taken approximately on the line 11 of FIGURE 2;

FIGURE 2 is a cross sectional view of FIGURE 1 taken approximately on the line 22 thereof and rotated clockwise through an angle of approximately 90; and,

FIGURE 3 is a schematic drawing showing the various hydraulic forces on the pump of FIGURE 1.

Referring now to the drawings wherein the showings are for the purposes of illustrating a preferred embodiment of the invention only and not for the purposes of limiting same, the figures show a hydraulic pump comprised of a housing H having an internal pumping cavity in which are mounted a plurality of pumping members defining a plurality of closed chambers which progressively increase and decrease in volume as the members move relative to each other. While such members may take a number of conventional forms, such as rotating cylinders with axially reciprocating pistons, or rotating vanes, or the like, in the embodiment of the invention shown, they comprise generally an externally-toothed gear member 11, an internally-toothed gear member 12, sealing and manifold members 13, 14, one engaging the right and the other the left hand axial faces of the gears 11, 12 respectively.

Pumping Members The gear member 11 is supported for rotation on the axis 15 of a shaft 16 and keyed thereto by a key 7 fitting into keyways 18, 19 on the gear 11 and shaft 16 respectively. The internally-toothed gear member 12 is supported for rotation about an axis 20 spaced from the axis 15 in a bearing member 17. The gear member 12 has one tooth more than that of the gear member 11 and these teeth are in sliding, sealing engagement so that as the gear members 11, 12 rotate they, along with the sealing and manifold members 13, 14 define a plurality of closed pumping chambers 21: and 21d which revolve on a closed path of movement and progressively increase in volume from a point A of minimum volume to a point B of maximum volume and then decrease to the point of minimum'volume A. The points A and B are in what may be termed the neutral plane through the two axes of rotation and it will be further noted that the gear teeth at the point A are in What may be termed closed mesh and at the point B at open mesh.

Housing The housing H in the embodiment shown is formed in two parts, namely, a main part 22 generally in the shape of a cup, and a closure part 23 removably positioned in the open end of the cup 22 by any suitable means, but preferably by means of threads 24. An O-ring 26 between opposed surfaces of the two parts and on the cavity side of the threads 24 provides a seal to prevent leakage of the hydraulic fluids longitudinally past the threads 24.

The main part 22 includes a side wall having a plurality of inwardly facing generally cylindrical surfaces 31, 32, 33 which are progressively larger in diameter reading from left to right. In a like manner, the closure part 23 has cylindrical surfaces 34, 34' which slidingly support a sealing ring 75.

Manifold Member The manifold member 14 is fixedly mounted in the housing in any suitable manner, but in the embodiment shown has a cylindrical surface 35 fitting within the surface 32 and an axially facing surface 35 bearing against the shoulder between the surfaces 32 and 31.

The shaft 16 extends into and is rotatably supported in the member 14, it being noted that by virtue of the design features of the pump that a simple inexpensive sleeve type bearing is employed.

The manifold member 14 defines, along with the cylindrical surface 31, a housing cavity 39 communicating with an inlet opening 36. This cavity 39 is at inlet pressure.

The manifold member extends beyond the cylindrical surface 32 and defines with the cylindrical surface 33 and the left hand axial end of the bearing member 17, a housing cavity 41 generally in the shape of a ring which communicates with an outlet opening 37.

The manifold member 14 has on its right hand axial end a surface 42 in sealing engagement with the left hand axial face of the gears 11, 12. An inlet manifold port 43 extending in an arcuate direction in the path of movement of the pumping chambers is formed in the sealing face 42 and extends axially through the manifold member 14 to communicate with the cavity 39.

Additionally, an outlet manifold port 44 is formed in the sealing surface 42 diametrically opposite from the inlet manifold 43 which also extends in an arcuate direction in the line of movement of the pumping chambers. A passage 46 which in this instance simply forms a radial extension of the manifold 44 communicates the port 44 with the housing cavity 41. This cavity 41 houses both gears 11, 12, the sealing member 13 and the bearing ring 17 and as an important part of the invention, is at discharge pressure.

The formation of the manifold ports 43, 44 in the surface 42 leaves portions of the sealing surface between the arcuate ends of the manifold and in the chamber path of movement which forms open mesh land 47 and closedmesh land 48, each having a line of movement width slightly greater than (by about 10) the line of movement width of one pumping chamber, that is to say, the line of movement width between the points of contact of adjacent teeth on one of the gears with adjacent teeth on the other of the gears.

Further, the hydraulic force on manifold member 14 creates a net axial thrust which is taken up on face 35 This force is used to seal the high pressure cavity from the low pressure cavity.

It will be appreciated that as the gears revolve, there may be a slight variation in the line of width movement of each chamber. The line of movement width referred to is that width existent at the instant when a chamber is adjacent a land.

Eccentric Bearing Mlember The eccentric bearing member 17, its mode of operation and its geometry are fully described and claimed in my copending application Serial No. 814,320 filed May 19, 1959, now Patent No. 3,034,448, and will not be further described herein.

Sealing Member The sealing member 13 is mounted on the shaft 16 for rotation therewith and as an important part of the present invention, is in effect, an integral part of the shaft 16. To achieve this effect, it may actually be made integral with the shaft 16, may be welded thereto, but in the preferred embodiment, a simple interference fit with the shaft 16 is employed.

It is further to be noted that the member 13 has a substantial axial and radial dimension, e.g. greater than at least one half the shaft diameter and preferably greater than the shaft diameter whereby the sealing member 13 contributes to the rigidity of the shaft 16 and in a like manner, any forces exerted on the member 13 act the same as though exerted on the shaft 16.

The sealing member 13 and shaft 16 are mounted for limited axial movement and for rotation relative to the housing by any suitable means, such as a roller bearing 54 consisting of an outer ring fitted into the housing H and a plurality of circumferentially spaced cylindrical rollers engaging an outer cylindrical surface 56 of the sealing member 13. Obviously, other types of bearings may be employed so long as they have the requisite radial load carrying ability and so long as the shaft 16 and the sealing member 13 may have a limited axial movement relative to the gear members 11, 12.

The sealing member 13 has a sealing surface 57 on one axial end in sealing engagement with the right hand axial faces of the gears 11, 12 and closes the right hand axial end of chambers 21. When the pump is operating, the high pressure fluid in the high pressure chambers exert a radially offset axial force indicated by the vector R to the right on the member 13.

The sealing member 13 also has axially facing pressure surfaces 84, 86 on the opposite axial end exposed to the pump discharge pressure in the cavity 41 which exerts an axial force indicated by the vector L to the left. The use of these forces will be explained hereafter.

It is to be noted that the area of the surface 86 exposed to the ldgh pressures in the housing cavity 41 is limited or restricted by means of the sealing ring 75.

Sealing Ring The sealing ring 75 forms an important part of the present invention. It has an axially slidable sealed relationship with the cylindrical surface 34 defining at least part of the shaft opening by means of an O-ring 76 mount ed on the outer surface of the ring. The ring 75 surrounds the shaft 16 and has a flange 78 extending radially in the direction of the high pressure i.e. radially outwardly in the embodiment shown. The ring 75 and flange 78 have a left hand axially facing sealing surface 78 formed on a radially outwardly extending flange 78 in pressure sealing relationship with the right hand axially facing surface 86 of the sealing member 13. The ring 75 thus defines an internal cavity 79 which is at inlet pump pressure, it being noted that the cavity 79 is communicated with the inlet through the keyway 19, a small counterbore 91 in the surface 57, an opposite keyway 92 in the gear 11, and a groove or passage 93 in the manifold member 14.

A compression spring 81 between the base of the housing part 23 and the right end of the sealing ring 75 biases the surfaces 78, 86 into a limited pressure engagement. It is to be noted that this spring also presses the sealing member 13 into engagement with the axial faces of the gear 11, 12 and presses the gear 11, 12 into pressure engagement with the sealing surface of the manifold member 14. The spring 81 is relatively weak and simply provides an initial force to maintain the various surfaces in pressure engagement When the pump is not operating or when it is started into operation. The principal sealing force is the hydraulically produced force above-referred to.

The surface 78 engaging the surface 86 seals the high pressure in the pump cavity 41 from the low pressure in the cavity 79. In accordance with the present invention, the sealing pressure between these two surfaces is provided by the high pressure in the cavity 41 exerting a left hand force on the right hand axially facing surface 87 of the flange 78 and in accordance with the invention, the area of the surface 87 is so proportioned that the force produced between the surfaces 78 and 86 is just equal to or slightly greater than the force of the pump discharge pressure tending to separate the surfaces.

The force tending to separate the surfaces is the product of the area of the surface 78 and/or integrated pressure between the outer and inner radial limits of the surface 73 which for design purposes may be taken as one half of the difference between the discharge pressure in the cavity 41 and the inlet pressure in the cavity 79. Accordingly, the area of the surface 87 which is exposed to the high pressure is made approximately equal to one half of the area of the surface 78 and the force created by the spring 81 may be relied upon to create a total force urging the surfaces 78, 36 into sealing engagement slightly greater than the total force tending to urge the surfaces apart.

This arrangement may be employed wherever an opening for a rotating shaft must be sealed against the leakage of high pressures.

A sealing arrangement results which has a minimum of leakage and a minimum of friction. The inside surface of the sealing ring contains a conventional low pressure lip type shaft seal 95.

Hydraulic Force Analysis The decreasing volume chambers 210. are all at the maximum high hydraulic pressures. These pressures exert hydraulic forces on all of the members with which they come in contact. Thus the hydraulic pressures exert forces over approximately one half of the radially facing surfaces of the inner gear member 11 which forces for the purposes of simplicity and analysis, may be integrated and considered as a single radial force U which in FIG- URES l and 3 is upward, acting at the circumferential mid point of the decreasing volume chambers.

The size of this force may be generally calculated as being equal to the difference between the output and inlet pressures multiplied by the cross sectional area of the gear 11.

This force U is transmitted from the gear 11 to the shaft 16 and thence to the housing through both the manifold member 14 which forms a bearing support for the shaft 16 and the roller bearing 54, creating a force D-1 generally on the midline of the bearing 14 and a force D-Z generally on the midline of the bearing 54, the line of action of which forces are spaced a dimension a and a dimension b respectively from the line of action of the force U. In the absence of the present invention and in accordance with known engineering principles, the size of the forces D-1 and D2 may be readily calculated inasmuch as they are inversely proportional to the spacing of their line of action from the line of action of the force U. It is to be noted that with an ordinary high pressure pump and in the absence of the present invention, it would be impossible to make the dimension a large enough or the dimension b small enough that the size of the force D1 will be small enough that an ordinary sleeve type bearing will be able to carry that portion of the force U represented as the force D1.

Thus, in accordance with the present invention, and as before pointed out, the sealing member 13 is in effect, an integral part of the shaft 16 and any hydraulic forces on the member 13 are the same as though they were exerted on the shaft 16.

The high pressure in the chambers 21a exert axial forces to the right on the surface 57 of the sealing member 13 which forces for the purposes of analysis, may be integrated as a single axial force R generally midway between the circumferential end of the high pressure chambers and spaced radially from the axis 15 a distance equal to the radial spacing of the center of the high pressure chambers, this distance being indicated by the dimension r. This force R is opposed hydraulically in accordance with the present invention by means of the high hydraulic pressures in the cavity 41 exerting hydraulic forces to the left on surface 84 and that portion of the surface 86 exposed to the high hydraulic pressures. As these forces are symmetrical about the axis 15, they may be considered as a single integrated force L acting on the axis of the shaft 16.

The size of the force L will generally be equal to the total axial cross-sectional area of the sealing member 13 minus the axial cross-sectional area of the sealing ring 75 plus one half of the area of the surface 78, all multiplied by the hydraulic pressures in the cavity 41 and will be slightly larger than R.

The forces R and L are radially offset one from the other and exert a turning moment on the sealing member 13 which in turn is transmitted to the shaft 16. This turning moment tends to move the end of the shaft 16 journaled in the bearing 14 in downward direction as viewed in FIGURE 3 with the result that the force D1 is correspondingly reduced, the force D-Z being correspondingly increased.

The effect of force R on the sealing member 13 is thus to reduce force Dl by an amount proportional to the relationships of the dimensions a, b, and r. Normally in a given pump, the dimension 1' is fixed by the diameter of the gear 11. The dimensions a and 12 however, are subject to certain variations and in particular, the bearing supporting the sealing member 13 can be adjusted axially to vary the dimension b. It is possible that dimension b be made small enough to have force D-l be reversed.

In the preferred embodiment however, it is preferred that the dimensions a, b and r be so proportioned that approximately 80% of the force U is transmitted to the housing as force D-2 through the bearing 54 and the remaining transmitted to the housing through the bearing 14 as force D-l.

With the force D4 of this magnitude, it is entirely possible to rotatingly support the left hand end of the shaft 16 using a conventional low cost sleeve type bearmg.

It is to be noted that this reduction in the size of D-l will follow for any relationship of a, b and r so long as the sealing member 13 is in effect an integral part of the shaft and is rigid enough to transmit the turning moment of the force R to the shaft.

Thus it will be seen that a hydraulic pump has been described which accomplishes all of the objectives heretofore set forth and others, in fact, hydraulic pumps constructed in accordance with the present invention can prove extremely satisfactory in rigorous tests both as to mechanical and volumetric eificiency as well as wear characteristics.

The invention has been described with particular reference to preferred embodiments. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification differing radically from the preferred embodiments described. It is my intention to include all such modifications and alterations insofar as they come within the scope of the claims.

Having thus described my invention, I claim:

1. In a hydraulic device comprised of: a housing having a radially-inwardly-facing surface defining a closed hollow interior, a shaft extending into said interior, a plurality of members rotatable with said shaft and relatively movable in said interior to define a plurality of axially-open-ended chambers which revolve on a fixedclosed path of movement, said chambers gradually increasing in volume after they pass a fixed point of minimum volume on said path of movement until they reach a fixed point of maximum volume on said path of movement and then gradually decreasing in volume until they reach said point of minimum volume, generally one half of said chambers being at high fluid pressure and generally the other half being at low fluid pressure, a sealing plate mounted on said shaft and having a sealing surface in sealing engagement with said members to close one axial end of said chambers, said high pressure exert ing a radially-offset axial force on said sealing plate, means exerting an opposing coaxial force on said plate, said forces exerting a moment on said plate tending to separate said sealing surface from said members adjacent said high pressure chambers, bearing means on both axial ends of said members rotatably supporting said shaft, said sealing plate and said shaft being effectively integral and being axially movable relative to at least one of said members, the improvement which comprises: said sealing plate having an axial length at least greater than onehalf of the diameter of said shaft whereby the moments created by the two axial forces on said plate are resisted by the combined strength of said shaft and said plate.

2. The improvement of claim 1 wherein said sealing plate and said shaft have an interference fit.

3. The improvement of claim 1 wherein said sealing plate and said housing for rotatably supporting said shaft and said sealing member in said housing.

4. The improvement of claim 1 wherein said sealing plate has an axial length at least greater than the diameter of said shaft.

5. In a hydraulic device comprised of a housing having a radially inwardly facing surface defining a closed hollow interior, a shaft extending into said interior, an externally-toothed gear mounted on said shaft for rotation therewith, an internally-toothed gear surrounding said externally-toothed gear, said gears having teeth in sliding sealing engagement to define a plurality of axially open-ended chambers which revolve on a fixed closed path of movement, said chambers gradually increasing in volume after they pass a fixed point of minimum volume on said path of movement until they reach a fixed point of maximum volume on said path of movement and then gradually decreasing in volume until they reach said point of minimum volume, generally one half of such chambers being at high fiuid pressure and generally the other half being of low fluid pressure, the gear members having axially facing sealing surfaces with each chamber having an opening through said surfaces, means engaging one of said axially facing sealing surfaces for closing one axial end of said chambers, a sealing plate having a sealing surface in sealing engagement with the opposite of said gear member sealing surfaces, first hearing means on the side of said gear member remote from said sealing plate rotatably supporting said shaft, second bearing means on the same side of said gear members as said sealing plate rotatably supporting said shaft, the improvement which comprises: said sealing plate being effectively integral with said shaft whereby the radially offset axial force of the pressures in said high pressure chambers on said sealing plate exert a turning moment on said plate which is transmitted to said shaft.

6. The improvement of claim 5 wherein said sealing plate has an axial facing pressure surface on the end opposite from said sealing plate sealing surface, said pressure surface being exposed to high pressures in the interior of said housing whereby to create an axial force on said plate.

7. In a hydraulic device comprised of a housing having a radially inwardly facing surface defining a closed hollow interior, a shaft extending into said interior from one end of said housing, an externally-toothed gear member mounted on said shaft for rotation therewith, an internally-toothed gear member surrounding said externallytoothed gear member and having gear teeth in slidable sealing relationship therewith to define a plurality of axially open-ended chambers which revolve on a fixed closed path of movement and progressively increase and decrease in volume, generally one half of said chambers being at a high fluid pressure and generally the other half being at low fluid pressure, means closing one axial end of said chambers, bearing means supporting said shaft at said axial end of said chambers for rotation in said housing, a sealing plate having a sealing surface in sealing engagement with the opposite end of said gear members to close the other axial end of said chambers, said high pressure exerting a radially offset axial force on said sealing plate, the improvement which comprises: said sealing plate having axially facing pressure surfaces on the end opposite from its sealing surface, means communicating the high fluid pressure to the interior of said pump and on said sealing plate pressure surfaces, said sealing plate and said shaft being etfectively integral, bearing means supporting said sealing plate for rotation in said housing, the axial spacing of said last-mentioned bearing means from the axial centerline of said chambers being so proportioned that the turning moment of the radially offset axial forces on said sealing plate reduces the radial loading on said first mentioned bearing means due to radial forces of said high fluid pressure on said externally-toothed gear.

References Cited in the file of this patent UNITED STATES PATENTS Read et al. Dec. 5, Hofmann Apr. 19, Northey Oct. 27, Buckbee Nov. 9, Thomson July 1, Kanuch et al. Apr. 28, Witchger Jan. 6, Parsons Sept. 1, Bodensieck Sept. 29, Quintilian Mar. 23, Miller et al Oct. 15, Crandall Dec. 8, Brundage Oct. 18, Murray Mar. 14,

FOREIGN PATENTS Germany Oct. 27, 

1. IN A HYDRAULIC DEVICE COMPRISED OF: A HOUSING HAVING A RADIALLY-INWARDLY-FACING SURFACE DEFINING A CLOSED HOLLOW INTERIOR, A SHAFT EXTENDING INTO SAID INTERIOR, A PLURALITY OF MEMBERS ROTATABLE WITH SAID SHAFT AND RELATIVELY MOVABLE IN SAID INTERIOR TO DEFINE A PLURALITY OF AXIALLY-OPEN-ENDED CHAMBERS WHICH REVOLVE ON A FIXEDCLOSED PATH OF MOVEMENT, SAID CHAMBERS GRADUALLY INCREASING IN VOLUME AFTER THEY PASS A FIXED POINT OF MINIMUM VOLUME ON SAID PATH OF MOVEMENT UNTIL THEY REACH A FIXED POINT OF MAXIMUM VOLUME ON SAID PATH OF MOVEMENT AND THEN GRADUALLY DECREASING IN VOLUME UNTIL THEY REACH SAID POINT OF MINIMUM VOLUME, GENERALLY ONE HALF OF SAID CHAMBERS BEING AT HIGH FLUID PRESSURE AND GENERALLY THE OTHER HALF BEING AT LOW FLUID PRESSURE, A SEALING PLATE MOUNTED ON SAID SHAFT AND HAVING A SEALING SURFACE IN SEALING ENGAGEMENT WITH SAID MEMBERS TO CLOSE ONE AXIAL END OF SAID CHAMBERS, SAID HIGH PRESSURE EXERT- 